Page 49 - Materials Australia - April 2019

FEATURE – Additive Manufacturing
Material
Density (g/cm3)
Elastic Modulus (GPa)
Co-Cr Alloy
>7
230
316L Stainless Steel
8.0
200
Titanium
4.51
110
Ti6Al4V
4.40
106
Hydroxyapatite
3.16
100
Cortical Bone
≈2.0
7-30
Table 1. Comparison of biomaterial properties.
the action of osetoeblasts (cells that facilitate the formation of new bone) and remodeling under the guise of osteoclasts (cells that breakdown bone for the purposes of repair and maintenance). The bioactive nature of hydroxyapatite allows bone to grow onto and (osseo)integrate it into the skeletal system.
While bioactive materials such as hydroxyapatite may seem like the ideal candidate for orthopaedic implants, their mechanical properties are not sufficient withstand the rigours of joint replacements such as knees and hips which can be subjected to repeated impact loadings during actions such as running and jumping.
Implant Designs
From a materials perspective, several materials may be incorporated into
a single implant. The main structure that provides strength and reliability
is made from metal. These are often coated in hydroxyapatite to improve their integration with bone. Articulating surfaces may incorporate wear resistant ceramics and sometimes UHMWPE.
While a number of metals are classified as biocompatible, titanium and the Ti6Al4V titanium alloy (borrowed for the aviation industry) are the favoured materials. They offer the lightest weight implant, while the stiffness is lower than that of the other alloys and closer to that of cortical bone. This lessens the liklihood of stress shielding, a process that can lead to bone resorption around an implant which can be detrimental.
Hydroxyapatite coatings are most commonly applied by plasma spraying. This involves injecting hydroxyapatite powder into a plasma flame which propels it at high velocity at a substrate. It relies on melting of the hydroxyapatite
and resolidification upon impact with
the metal substrate. The problem is that hydroxyapatite is thermally unstable
and decomposes into other calcium phosphates, such as ß-TCP (Ca3(PO4)2) at plasma spraying temperatures. ß-TCP can be resorbed under physiological conditions which can compromise the integrity of the coating, possibly leading to delamination and implant loosening. Careful control of plasma spray parameters is required to produce a structurally sound coating while minimising decomposition.
Porous metal coatings, such as beads, can also be incorporated into the implant design. Either hydroxyapatite-coated or uncoated, these allow bone to grow onto the network of pores which can lead
to better implant integration and load distribution.
Many orthopaedic implants take the form of joints such as hips and knees
and have articulating surfaces. Constant movement necessitates these surfaces to be wear resistant to ensure longevity and prevent the formation of wear particles that could affect surrounding tissues as well as being low friction for comfort.
As with the structural component, the ability to withstand impact loads is also
a requirement. Metals were traditionally used here, but nowadays ceramics such
as alumina (Al2O3) and PSZ (partially stabilised zirconia, ZrO2) and UHMWPE are the most popular materials, with different combinations being employed by different manufacturers.
The mechanical designs are often the results of detailed FEA (Finite Element Analysis) studies. FEA is used to refine the design not only for mechanical reliability, but also for biomechanics including recipient mobility and load.
Recent Developments
The long-term success of an implant can be dependent on several factors. One of these is the accuracy of the surgery. Computer- aided and robotic assisted surgery has increased the accuracy of alignment of bone cuts and implant alignment. They
can also significantly reduce the level of invasiveness of the surgery which can decrease the recovery time. These are all excellent patient outcomes that can result a reduced likelihood of surgical revision at a later date.
Additive manufacturing has also been responsible for recent developments in this field. Previously, surgeons had to choose from a range of standard size implants. Using modern medical imaging techniques to non-invasively look inside a patient, custom designed implants can be produced that cater exactly to a patient’s needs. Additive manufacturing also provides implant design engineers new levels of design freedom with the ability to produce new implant designs. This has also allowed smaller players to enter the market.
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